Catalytic reforming or hydroreforming is a well known process in the refining industry whose object is generally to improve the octane number of heavy gasolines or of straight-run gasolines and/or to produce aromatic hydrocarbons. This operation must be achieved on an industrial scale with the highest possible yield to the desired products: basic components for motor fuels or aromatic compounds for the petrochemical industry. Usually the yield is measured by the product amount of gasoline containing hydrocarbons of five carbon atoms or more, commonly called C.sub.5.sup.+ gasoline cut, expressed in proportion to the introduced charge.
Presently, another factor, directly associated to the latter, has an increasing importance: the hydrogen production. The desired object is to obtain the highest possible yield of both C.sub.5.sup.+ cut and hydrogen.
In the near future, the application of antipollution standards will lead to a decrease of the lead concentration in gasolines so as to avoid, in the long term, the presence of lead organic additives in motor fuels for spark-ignition engines. This regulation obliges the refiners to increase the severity of the operating conditions of reforming units in order to comply with the quality criteria required for satisfactory running conditions of car engines. Moreover, refining restructuration which more and more favors conversion and hydrotreatment units will substantially increase the hydrogen requirements of the refineries. These additional hydrogen requirements will have to be satisfied to a large extent by catalytic reforming or hydroreforming processes whose operating conditions are so adjusted as to meet this demand.
The greater severity of the operating conditions in industrial units, for example decrease of the operating pressure and, if necessary, increase of the temperature, is thermodynamically favorable to reactions producing the desired products (aromatics and hydrogen) but is highly detrimental to the stability of the catalysts. As a matter of fact, this increased severity results in an increase of the rate of hydrocarbon deposit, as coke, on the catalyst, which is responsible for a more rapid loss of catalyst activity.
This evolution requires a more frequent regeneration of catalysts and, accordingly, a decrease of operation time and hence of total treatment capacity of industrial units. It is clear that any improvement in the catalyst stability will result in a decrease of the regeneration frequency or, otherwise stated, in a lengthening of the operation time between two successive regeneration steps, which constitutes significant progress toward a more rational use of industrial units.
The catalysts used in catalytic reforming generally comprise, as an essential element, alumina containing a noble metal from group VIII of the periodic classification of elements, generally platinum for first generation catalysts such for example as those catalysts disclosed by V. HAENSEL in OIL AND GAS JOURNAL, vol. 48 No. 47 pages 82 and following, 1950. A second generation of catalysts has subsequently appeared, where a so-called promoter is added to the group VIII metal. Examples of the most usual promoters are rhenium (U.S. Pat. No. 3,415,737), tin (U.S. Pat. No. 3,700,588), indium and thallium (U.S. Pat. No. 2,814,599). Substantial improvements in stability and/or selectivity on the so-called first generation catalysts were obtained by the addition of these different promoters. The addition of two promoter elements has also been described as substantially improving the stability and/or selectivity. A catalyst comprising an alumina carrier, platinum, rhenium and tin is for example disclosed in U.S. Pat. No. 3,702,294.
These various catalysts have been tested and/or used over long periods, for example as long as one year or even longer. Each catalyst formula simultaneously offers advantages and suffers from disadvantages:
platinum-rhenium catalyst has an excellent stability but does not give the maximum selectivity for the production of high grade gasolines, PA1 platinum-tin, platinum-indium or platinum-thallium catalysts provide for an excellent selectivity but their stability is much lower than that of the platinum-rhenium catalyst, PA1 the catalysts containing three metals (e.g. platinum, rhenium and tin) have neither the stability of platinum-rhenium nor a selectivity equivalent to that of platinum-tin, platinum-indium or platinum-thallium. PA1 *r.sub.a &gt;r.sub.b .gtoreq.r.sub.c preferably with a r.sub.a value from 1.6:1 to 4:1 and more preferably from 2:1 to 3:1 and r.sub.b and r.sub.c values preferably ranging from 0.5:1 to 1.5:1 and more preferably from 0.8:1 to 1.1:1. PA1 *r.sub.a &gt;r.sub.b &gt;r.sub.c, with the preferred values as above indicated. PA1 *r.sub.a .gtoreq.r.sub.b and r.sub.b .gtoreq.r.sub.c, r.sub.a being always greater than r.sub.c and r.sub.a ranging preferably from 1.6:1 to 4:1 and more preferably from 2:1 to 3:1, whereas r.sub.b preferably ranges from 1:1 to 2:1 and r.sub.c preferably from 0.8:1 to 1.1:1.
In order to improve the stability of the catalysts containing platinum and rhenium, deposited on alumina, an increase of the atomic or weight ratio between the rhenium amount and the platinum amount contained in the catalyst has been proposed and disclosed in the Belgian Pat. No. 875,386 (corresponding to GB-A-No. 2018278).
In this Belgian patent, the described catalyst composition has a rhenium-to-platinum ratio by weight from 2 to 5 and preferably from 2.25 to 4. The use of catalysts of increased rhenium content will result in a relative lengthening of the cycle, which may exceed by 65% the cycle involving the use of catalysts containing substantially the same amount of metals: platinum+rhenium. Nevertheless, the use of catalysts of increased rhenium content generally results in a decrease of the C.sub.5.sup.+ gasoline yield by more than 1% by weight. Moreover, the positive effect of an increased rhenium content on the stability is only observed when the sulfur content of the treated naphtha is lower than 0.5 ppm by weight.
In order to avoid this selectivity decrease, due to the use of catalysts of higher rhenium content in all the reactors, it has been suggested to use a platinum-rhenium catalyst whose rhenium-to-platinum ratio by weight is lower than or equal to about 1, in the one or more first reactors and a platinum-rhenium catalyst having a rhenium-to-platinum ratio by weight higher than or equal to about 2, in the one or more last reactors. Such arrangements are for example disclosed in the following documents: French Pat. No. 2 467 236, U.S. Pat. Nos. 4,427,533 and 4,436,612. They provide for an equivalent or higher stability and a slightly improved selectivity to C.sub.5.sup.+ gasoline cut as compared with the catalyst containing platinum and rhenium in a ratio by weight lower than or equal to about 1, used in all the reactors.
Furthermore, according to the European patent application No. 153 891, the use in the first reactor of a catalyst having a better stability than that of the catalysts used in the other reactors results in a substantial improvement of the stability and selectivity of all of the catalyst beds contained in the different reactors of the unit.